[tools][gen] Describe how to regenerate files in the header

Change-Id: I9b0b9e4ecad25122b4b046c1283333aee48ee246
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/147801
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
diff --git a/src/tint/BUILD.cmake b/src/tint/BUILD.cmake
index 2199406..77ab7d4 100644
--- a/src/tint/BUILD.cmake
+++ b/src/tint/BUILD.cmake
@@ -13,11 +13,12 @@
 # limitations under the License.
 
 ################################################################################
-# File generated by tools/src/cmd/gen
-# using the template:
+# File generated by 'tools/src/cmd/gen' using the template:
 #   tools/src/cmd/gen/build/BUILD.cmake.tmpl
 #
-# Do not modify this file directly
+# To regenerate run: './tools/run gen'
+#
+#                       Do not modify this file directly
 ################################################################################
 
 include(api/BUILD.cmake)